The kidneys are two bean-shaped organs that perform several essential jobs that keep the body’s internal environment stable. Their best-known function is making urine, but urine production is only one part of their work. The kidneys continuously filter the blood, remove metabolic wastes, control the amount of water and electrolytes in the body, help regulate blood pressure, maintain the blood’s acid-base balance, and produce hormones that support red blood cell production and bone health.
Because the kidneys work around the clock and adjust their activity according to the body’s needs, they are central to maintaining a stable internal environment, a process called homeostasis.
Where the kidneys are and how they work
Most people have two kidneys, located toward the back of the abdomen, one on each side of the spine. They sit below the lower ribs and behind the abdominal organs. Each kidney contains roughly a million microscopic filtering units called nephrons.
A nephron has two major functional parts. The glomerulus is a tiny cluster of blood vessels where fluid and small dissolved substances are filtered out of the blood. The renal tubule then modifies that filtered fluid by taking useful substances and water back into the bloodstream while leaving behind substances that the body needs to eliminate.
Blood enters the kidneys through the renal arteries. After filtration and extensive adjustment within the nephrons, the cleaned blood leaves through the renal veins. The material that remains in the tubules becomes urine, which drains through the ureters into the bladder and eventually leaves the body through the urethra.
The kidneys therefore do more than simply strain the blood. They filter, selectively reclaim, secrete, and concentrate substances according to what the body needs.
The kidneys remove waste from the blood
Cells constantly produce waste as they use nutrients and carry out normal chemical reactions. The kidneys help remove many of these unwanted substances from the bloodstream.
One important waste product is urea, which forms when the body breaks down proteins. The kidneys also eliminate excess creatinine, a waste product related to normal muscle metabolism, along with various other substances that the body needs to dispose of.
Some substances are removed from the blood by filtration at the glomerulus. Others are actively secreted into the renal tubules. The kidneys can also alter how much of a substance is reabsorbed, allowing the body to retain something useful rather than lose it in urine.
This careful handling is why healthy urine contains waste and excess substances but normally retains very little of many valuable molecules, such as glucose and amino acids.
They control how much water the body retains
The kidneys play a major role in maintaining the body’s water balance.
As blood is filtered, a large amount of water initially enters the nephron. Most of that water is subsequently reabsorbed into the bloodstream rather than lost in urine. The amount reabsorbed changes depending on the body’s needs.
When the body needs to conserve water, the kidneys can produce a smaller volume of more concentrated urine. When there is plenty of water available, they can excrete a larger volume of more dilute urine.
A hormone called antidiuretic hormone (ADH) is particularly important in this process. ADH signals parts of the kidney’s tubules to become more permeable to water, allowing more water to return to the bloodstream.
This system helps keep the concentration of dissolved substances in body fluids within a relatively narrow range.
They keep electrolytes in balance
Electrolytes are minerals that carry an electrical charge when dissolved in body fluids. Important examples include sodium, potassium, calcium, chloride, and phosphate.
These substances are essential for nerve signals, muscle contraction, heart function, fluid balance, and many other processes. Too much or too little of an electrolyte can interfere with normal body function.
The kidneys regulate electrolytes by filtering them and then selectively reabsorbing or excreting them. For example, the kidneys adjust potassium excretion according to the body’s needs, helping prevent potassium levels in the blood from becoming dangerously high or low.
Sodium regulation is especially important because sodium strongly influences the amount of water in the bloodstream and other body fluids. By controlling sodium and water together, the kidneys contribute to maintaining appropriate blood volume and pressure.
They help regulate blood pressure
The kidneys influence blood pressure through both fluid balance and hormonal signaling.
When the body retains more sodium, it generally retains more water as well. This can increase the volume of fluid circulating through the blood vessels. The kidneys also participate in a hormonal system called the renin-angiotensin-aldosterone system, or RAAS.
When the kidneys detect conditions such as reduced blood flow or reduced sodium delivery, specialized kidney cells can release renin, an enzyme that initiates a series of reactions leading to the production of angiotensin II. Angiotensin II helps raise blood pressure and promotes mechanisms that conserve sodium and water.
The kidneys therefore both respond to changes in blood pressure and help create longer-term changes in blood pressure regulation.
They maintain the blood’s acid-base balance
Normal body chemistry depends on keeping the blood within a tightly controlled acid-base range. The kidneys are one of the body’s two major systems for maintaining this balance; the lungs are the other.
The kidneys help control blood acidity by regulating hydrogen ions and bicarbonate, an important chemical buffer. They can excrete hydrogen ions into urine and reclaim or generate bicarbonate as needed.
This process is slower than the lungs’ response to changes in carbon dioxide, but it provides an important long-term mechanism for maintaining the body’s acid-base balance.
They produce hormones that affect other organs
The kidneys also function as endocrine organs, meaning they make substances that act as hormones or hormone-related signals elsewhere in the body.
One of the most important is erythropoietin, often called EPO. When the kidneys detect that tissues are receiving too little oxygen, they can increase production of EPO. EPO signals the bone marrow to make more red blood cells, which carry oxygen throughout the body.
The kidneys also help activate vitamin D. They convert vitamin D into its biologically active form, calcitriol, which helps the body regulate calcium and phosphate and supports normal bone metabolism.
These functions help explain why kidney disease can affect systems far beyond urine production. Reduced kidney function can interfere with red blood cell production, mineral balance, blood pressure regulation, and bone health.
The kidneys carefully decide what leaves the body
A useful way to understand kidney function is to think of urine formation as a multistep process rather than simple filtration.
First, blood passes through the glomerulus, where water and many small dissolved substances move into the nephron. Blood cells and most large proteins normally remain in the bloodstream.
Next, the renal tubules selectively reabsorb substances the body wants to keep. Water, sodium, glucose, amino acids, and other useful substances can be returned to the blood in carefully regulated amounts.
The tubules also secrete certain substances from the blood into the tubular fluid. The final composition of urine is therefore the result of filtration plus selective reabsorption and secretion.
This allows the kidneys to perform two jobs at once: eliminate unwanted material and prevent the unnecessary loss of substances the body needs.
What happens when the kidneys cannot work properly?
When kidney function declines, wastes and excess fluid can accumulate in the body. Problems with electrolyte levels, acid-base balance, blood pressure, hormone production, and mineral metabolism can also develop.
Kidney problems range from temporary injuries to long-term chronic kidney disease, in which kidney function gradually becomes impaired. Early kidney disease may cause few or no noticeable symptoms, which is one reason kidney function is often evaluated with blood and urine tests in people who are at increased risk.
Severe loss of kidney function can eventually require dialysis, which uses a treatment system to perform some of the kidneys’ waste-removal and fluid-regulation functions, or a kidney transplant.
The kidneys are therefore not simply the organs that make urine. They are sophisticated regulators of the body’s chemistry, continuously adjusting what stays in the bloodstream and what leaves the body. Their work helps keep fluid levels, electrolytes, acidity, blood pressure, red blood cell production, and mineral balance within ranges that allow the rest of the body to function normally.
